This invention relates generally to injection molding and more particularly to a mold that provides improved de-gating of parts molded from elastic material.
In injection molding, a thermoplastic material is forced under pressure into a mold cavity that defines the shape of the part being molded. A nozzle, providing the source of plastic material, communicates with the mold cavity through a sprue passage joining with one or more runner passages, in turn connecting to the mold cavity at corresponding gates. Generally, the gates are somewhat narrower than the runner passages to allow the sprue and runners (referring respectively to plastic remaining in the sprue and runner passages at the completion of the molding) to be broken away from the molded part at the gate in a process termed “de-gating”.
De-gating may occur as a separate process after the part is ejected from the mold or automatic de-gating may be accomplished with certain mold designs. For example, a tunnel-gate, also known as a “hook” or “submarine” gate, may be used to capture the runner in one part of the mold portion, so that when the molded part is ejected, the retained runner is pulled from the part. The tunnel-gate takes its name from the fact that the runner passage, which normally extends along the part line of the mold, “tunnels” below the part line into one mold part and then “hooks” upward into the mold cavity. The runner is removed from the mold with a slight twisting and/or flexing of the runner.
Automatic de-gating using, for example, a tunnel-gate, is often unsuccessful with elastic materials. Elastic materials, by stretching, dissipate the de-gating force developed as the part is ejected. The inability to de-gate such parts without a secondary operation, increases the cost of molding parts using elastic materials.